commentary about extern register

R=rsc
DELTA=16  (12 added, 0 deleted, 4 changed)
OCL=33030
CL=33035

diff --git a/src/pkg/runtime/runtime.h b/src/pkg/runtime/runtime.h
index 88e5161392a4041c84d0f86c605d86b3435f5bc8..c9b6a26118efb8af982c475c4af7067cd586fe19 100644 (file)
--- a/src/pkg/runtime/runtime.h
+++ b/src/pkg/runtime/runtime.h
@@ -64,10 +64,22 @@ typedef     struct  hash            Hmap;
 typedef        struct  Hchan           Hchan;
 
 /*
- * per cpu declaration
+ * per-cpu declaration.
+ * "extern register" is a special storage class implemented by 6c, 8c, etc.
+ * on machines with lots of registers, it allocates a register that will not be
+ * used in generated code.  on the x86, it allocates a slot indexed by a
+ * segment register.
+ *
+ * amd64: allocated downwards from R15
+ * x86: allocated upwards from 0(FS)
+ * arm: allocated upwards from R9
+ * 
+ * every C file linked into a Go program must include runtime.h
+ * so that the C compiler knows to avoid other uses of these registers.
+ * the Go compilers know to avoid them.
  */
-extern register        G*      g;      // R15
-extern register        M*      m;      // R14
+extern register        G*      g;
+extern register        M*      m;
 
 /*
  * defined constants